I have always had cats dude
OK, are they Yowling as in "daddy I don't like this" or are they Growling as in "GTFO"?
After hearing that though, sounds like Fisher is trying to find his own spot, and if they have taken the "daddy spot" (your bed) away from him as an option, it doesn't surprise me he wants to find somewhere else he feels safe and growls at interlopers. I would say Fisher is not the only one in need of a good spraying/nerfing.
Have you got 2 rooms where you can lock the 2 "combatants" away from eachother if the do fight? If you do, lock em up for a bit, and let the 3rd one have full run of the house, jealousy is not just a human trait